Process for making an anti-reflective coating composition and a porous coating made therefrom

ABSTRACT

The invention relates to a process of making an anti-reflective coating composition comprising the steps of 1) preparing an oil-in-water emulsion by mixing an apolar organic compound A; a cationic addition copolymer C as emulsion stabilizer; and aqueous medium of pH 2-6; at a mass ratio C/A of 0.1 to 2, to result in 1-50 mass % (based on emulsion) of emulsified droplets of average particle size 30-300 nm; and 2) providing an inorganic oxide shell layer to the emulsified droplets by adding to the emulsion obtained in step 1) at least one inorganic oxide precursor, to result in organic-inorganic core-shell nano-particles with mass ratio core/shell of from 0.2 to 25. An advantage of this process is that the dispersion of nano-particles obtained is stable under different conditions, and allows altering its concentration and solvent system, and addition of different binders and auxiliary components. The invention also relates to a coating composition as obtained with said process, and to a process of applying a porous anti-reflective-coating on a substrate using such composition, and to the resulting coated substrate.

The invention relates to a process for making an anti-reflective (AR)coating composition comprising the steps of preparing an aqueousdispersion of organic-inorganic core-shell nano-particles, andoptionally adding an inorganic or organic polymeric or polymerisablecompound as binder.

The invention also relates to a coating composition as obtained withsaid process, and to a process of applying a porous anti-reflectivecoating (ARC) on a substrate using such composition, and to theresulting coated substrate.

A typical example of an ARC is a thin layer of porous inorganic oxide—for example a layer of less than 0.2 μm thickness—which substantiallyconsists of an inorganic oxide like silica and has certain porosity.Such coatings may be applied to a transparent substrate to reduce theamount of light being reflected from its surface, i.e. from theair-substrate interface, and thus increase the amount of light beingtransmitted through the substrate. Such coatings can be used as singlelayer or as part of a multi-layer coating (or coating stack). Typicalsingle layer ARCs based on thin porous silica layers have been describedin e.g. EP0597490, U.S. Pat. No. 4830879, U.S. Pat. No. 5858462,EP1181256, WO2007/093339, WO2008/028640, EP1674891, WO2009/030703,WO2009/140482, US2009/0220774, and WO2008/143429.

A single layer ARC on a transparent substrate typically should have arefractive index between the refractive indices of the substrate andair, in order to effectively reduce the amount of light reflected. Forexample, in case of a glass with refractive index 1.5 the AR layertypically should have a refractive index of about 1.2-1.3, and ideallyof about 1.22. A porous silica (or other inorganic oxide) layer havingsufficiently high porosity can provide such a low refractive index andfunction as AR coating, if its layer thickness is about ¼ of thewavelength of the light; meaning that in the relevant wavelength rangeof 300-800 nm the thickness preferably is in the range 70-200 nm.

Optimum porosity and pore size in an ARC is not only depending on thecoating layer thickness, but also on other desired performancecharacteristics. For example, pore size should not be too large, tominimise light scattering and optimise transparency. On the other hand,if the layer contains very small pores, this may result —under ambientuse conditions—in non-reversible moisture up-take via capillarycondensation; affecting refractive index and making the coating layermore prone to fouling. Such capillary condensation effects have beenreported for so-called meso-porous silica, especially having pores inthe range 1-20 nm. Porosity is needed to reduce refractive index, buttoo high porosity level may deteriorate mechanical strength of thecoating, e.g. reduce (pencil) hardness and abrasion resistance.

By inorganic oxide precursor is herein meant a compound that containsmetal and can be converted into a metal oxide for example by hydrolysisand condensation reaction.

Porous AR coatings can be made from a solvent borne coating compositioncomprising an inorganic or organic binder material and a pore formingagent. In case of an inorganic binder, for example based on an inorganicoxide precursor, typically a sol-gel process is used for making a(porous) inorganic oxide layer, wherein a precursor compound in solutionor colloid (or sol) form is formed into an integrated network (or gel)of either discrete particles or network polymers. In such process, thesol gradually evolves to a gel-like diphasic system containing both aliquid and solid phase. Removing remaining liquid (drying) is generallyaccompanied by shrinkage and densification, and affects finalmicrostructure and porosity. Afterwards, a thermal treatment at elevatedtemperature is often applied to enhance further condensation reactions(curing) and secure mechanical and structural stability. Typicalinorganic oxide precursors used are metal alkoxides and metal salts,which can undergo various forms of hydrolysis and condensationreactions. Metal is understood to include silicon within the context ofthis description.

Such AR coating composition contains solvent and organic ligands fromorgano-metallic inorganic oxide precursors, which compounds as such willinduce some porosity to the inorganic oxide layer; but typically withpores smaller than 10 nm. The further presence of a pore forming agentin the coating composition will help in generating suitable porositylevel and pore sizes in the final AR layer to provide the desiredrefractive index. Suitable pore forming agents, also called porogens,known from prior art publications include organic compounds, like higherboiling solvents, surfactants, organic polymers, and inorganic particleshaving sub-micron particle size, including porous particles andorganic-inorganic core-shell nano-particles.

Use of porous or hollow nano-particles in a binder or matrix materialrepresents an elegant way to control porosity level and pore sizes in aporous ARC layer. Various different synthetic strategies for makinghollow inorganic particles can be distinguished, as for exampledescribed in Adv. Mater. 2008, 20, 3987-4019. A typical approach appliesa micro- or nano-sized organic structure in a solvent system as atemplate or scaffold for forming an inorganic oxide outer layer (alsoreferred to as coating with inorganic oxide), resulting in hybridorganic-inorganic core-shell (or coated) nano-particles as intermediateproduct. Shell layers comprising silica are generally made with asol-gel process based on the so-called Stöber method, wherein an alkoxysilane is hydrolysed and condensed in water/alcohol mixtures containingammonia.

A coating composition comprising pre-fabricated hollow inorganicnano-particles and a matrix-forming binder based on silica precursors isfor example described in EP1447433. A disadvantage of this approach isthat porous or hollow nano-particles, which can be re-dispersed in aliquid composition, are difficult to make.

Many documents address using organic-inorganic core-shell nano-particleswith an organic polymer core as an alternative to making an AR coatingcomposition. After applying and drying such coating composition on asubstrate, the organic core can be removed from the coating by variousmethods; for example by exposing the coating to a solvent for thepolymer and extracting it from the coating. Alternatively organicpolymer can be removed during thermally curing the coating, for exampleat temperatures above the decomposition temperature of the polymer (i.e.by pyrolysis or calcination). Suitable temperatures are from about 250to 900° C. A combined treatment of dissolving and degrading/evaporatingthe polymer may also be applied.

WO2008/028640 represents an example of a publication that describesapplying such hybrid core-shell nano-particles with a sacrificialorganic polymer core. In this document cationic polymer micelles orcationically stabilised polymer latex particles are used as organictemplate for making hybrid core-shell nano-particles and AR coatingcompositions comprising said core-shell particles and inorganic oxideprecursor as binder. Thermally curing an applied layer results in aporous coating with in situ formed hollow particles embedded in thebinder, wherein pore size is dependent on the size of the polymertemplate used for making the core-shell particles. A disadvantage ofusing nano-particles with a polymeric core is that for effective removalexposure to high temperatures may be needed, which limits its use withthermally sensitive substrate like thermoplastics.

JP 2011111558 discloses a coating composition where particles are basedon a polymer emulsion particle based on polymerized vinyl monomer with aspecific functional group, hydrolysable silicone compound, emulsifierand water.

JP2008274261 discloses a coating composition for forming a hollowsilicone particle film. Examples show that the hollow silicone particlesare prepared by first making a core of polymer particles using anionicor non-ionic emulsifiers and thereafter silicone precursors are added toform the shell, whereafter the core is removed by extraction or heating.A potential solution to above indicated thermal restrictions wouldinclude core-shell particles that have a more volatile core, for examplecomprising an organic compound with a boiling point below 300° C.Numerous research groups have published on synthesis and properties ofhybrid core-shell particles and inorganic hollow nano-particles; see forexample reviews such as Adv. Mater. 2008, 20, 3987-4019. Preparation ofstable dispersions of suitable core-shell nano-particles based on lowmolar mass organic templates and having particle size well below about500 nm appears however rather difficult.

Therefore, there remains a need in industry for an anti-reflectivecoating composition based on hybrid organic-inorganic core-shellnano-particles, which composition can be made into an ARC at relativelylow temperatures, for example on thermally sensitive substrates.

It is thus an objective of the present invention to provide such animproved AR coating composition and a process of making it.

The solution to above problem is achieved by providing the process asdescribed herein below and as characterized in the claims.

Accordingly, the present invention provides a process of making ananti-reflective coating composition comprising the steps of

-   1) Preparing an oil-in-water emulsion by mixing

an apolar organic compound A;

a cationic addition copolymer C as emulsion stabilizer; and

aqueous medium of pH 2-6;

at a mass ratio C/A of 0.1 to 2, to result in 1-50 mass % (based onemulsion) of emulsified droplets of particle size 30-300 nm; and

-   2) Providing an inorganic oxide shell layer to the emulsified    droplets by adding to the emulsion obtained in step 1) at least one    inorganic oxide precursor, to result in organic/inorganic core-shell    nano-particles with mass ratio core/shell of from 0.2 to 25.

With the process of the invention it is found possible to make a coatingcomposition comprising core-shell particles based on emulsified dropletsthat contain a liquid apolar organic compound and having an particlesize in the range of 30 to 300 nm. This particle size can be controlledby the type of organic compound and the monomer composition of thecationic copolymer, and/or by selecting conditions like temperature, pH,aqueous medium composition, and to a lesser degree stirring rate. Afurther advantage of the present process is that the dispersion ofcore-shell nano-particles obtained is stable under different conditions;increasing its shelf life or storage time, and allowing for examplealtering its concentration and solvent system, and addition of differentbinders and auxiliary components. The coating composition obtained withthe process according to the invention can be advantageously used formaking porous ARCs at a wide range of temperatures on differentsubstrates, including thermoplastic substrates that have low temperatureresistance; because the organic compound can be easily removed from thecore-shell particles by solvent extraction or evaporation at relativelylow temperature, and low temperature curable binder can be used.

The process according to the invention comprises a step 1) wherein anapolar organic compound A; a cationic addition copolymer C; and anamount of an aqueous medium of pH 2-6 are mixed at a mass ratio C/A of0.1 to 2 to prepare an oil-in-water emulsion. The apolar organiccompound A used herein is typically a liquid at temperatures above 0°C., preferably above 10 or 20° C. in order to make an oil-in wateremulsion. Compound A preferably retains liquid character underconditions of making the emulsion, and during subsequent process andstorage steps. On the other hand, it is an advantage that during makingan ARC from the composition, especially during a curing step, compound Acan be easily removed from the coating by evaporation to createporosity. Therefore, compound A has preferably a boiling point of atleast 30, 40, 60, 80, or 100° C., and of at most 350, 300, 250, 200 or150° C.

Compound A used in the process according to the invention has an apolar(or hydrophobic) character, meaning that it has solubility in water atroom temperature of at most 5 kg/m³ and can form a separate phase fromwater. Compound A is preferably a compound that can be dispersed inwater in the presence of copolymer C as described hereinafter.Preferably compound A has a solubility in water at room temperature ofat most 4, 3, 2, or 1 kg/m³.

It is highly preferred that compound A is non-polymeric. Bynon-polymeric is herein understood that A is not build from more than 2repeating monomer units as it was found that particularly forapplications where high temperature curing is not acceptable, it mayrequire extended time to extract or evaporate compound A from thecore-shell nano particles when compound A is a polymer.

Compound A preferably is inert with regard to other components used inthe process or composition of the invention. In a special way ofoperating the process according to the invention compound A may bereactive with the binder that is added in step 3) of the process asdescribed hereafter; especially in case the aqueous medium of the finalcoating composition differs from that in step 1) and contains e.g. moreorganic solvent, and compound A shows enhanced solubility therein ascompared to the aqueous medium during making the emulsion. In such casecompound A may be partially extracted from the dispersed core-shellparticle into the medium that may also contain binder. Compound A mayfor example be an unsaturated compound that can co-react with organicpolymerisable binder during curing of a coating layer.

Examples of apolar compound A include hydrocarbon compounds, fattyacids, alcohols, esters, ethers, vinyl compounds, and the like. Suitableexamples include cyclohexane, decane, toluene, trimethyl benzene,isoamyl acetate, C8-C22 alcohols, styrene, alkyl (meth)acrylates,butanediol dimethacrylate, hexanediol dimethacrylate, and the like.

In the process according to the invention one compound may be used ascompound A, but also a mixture of compounds as defined above may beapplied. It is also possible to use a compound A wherein a minor amount,for example less than 4, or 2 mass %, of a hydrophobic compound notsatisfying above definition is dissolved, which may aid in dispersingcompound A. An example of a suitable mixture is cyclohexane containing 1mass % of heptadecane.

In step 1) of the process according to the invention an oil-in-wateremulsion is prepared of compound A and a cationic addition copolymer C.Copolymer C comprises at least one monomer unit having a cationic chargeand at least one neutral or non-ionic monomer, preferably an apolarmonomer of which the corresponding homopolymer shows limited or nosolubility in water. Overall, the cationic copolymer should have apositive zeta potential. Preferably, cationic copolymer C is not readilysoluble in water, but tends to form colloidal aggregates, which enhancesits functioning as emulsion stabilizer. The copolymer may be a random,but also a block copolymer, and may comprise styrenic, acrylic,methacrylic, olefinic, and/or vinylic comonomers. Within the context ofthis application all these monomers are together referred to asethylenically unsaturated monomers or vinyl monomers; that is includinga.o. methacrylates which comprise a methyl-vinyl group. Acrylic andmethacrylic compounds are together typically referred to as(meth)acrylic monomers in the art. The addition copolymer canadvantageously be made using various polymerisation techniques as knownto a skilled person, and from a great number of suitable vinyl monomers;offering a wide range of compositions for the copolymer. Suitableexamples include bulk, solution and emulsion polymerisations usingradical initiators. The copolymer is preferably provided as dispersionin aqueous medium, which may have resulted from an emulsionpolymerisation of selected monomers, but also from polymerisation inorganic solvent followed by dispersing the copolymer obtained in aqueousmedium and optionally neutralising non-ionic functional groups; as isknown in the art.

JP2011111558 does not disclose the presence of a component A in the corethat fall within or even function as the component A disclosed in thepresent invention, but only the use of a polymer core. Furthermore, theemulsion stabilizer C in the form of a cationic addition copolymer isalso not disclosed.

JP208274261 suggests in the description that solvent may be present withthe polymer or even replace the polymer in the core of the core-shellparticles of JP208274261. However, there is no enabling disclosure ofeven the presence of solvent in the core and particularly no enablingdisclosure that it would be possible to prepare a core-shell nanoparticle without the presence of a polymer of the types indicated inJP208274261 in the core. Furthermore, JP208274261 does not disclose theuse of emulsion stabilizer C in the form of a cationic additioncopolymer.

In the process according to the invention copolymer C is preferably acopolymer obtained from

-   -   at least one cationic or basic monomer (M1), including compounds        with a pending group that can combine with a proton to later        form a cationic group; like monomers with a tertiary amine        group;    -   at least one neutral or non-ionic monomer (M2); preferably an        apolar monomer of which the corresponding homopolymer is not        readily soluble in aqueous medium; and    -   optionally at least one polar, anionic or acidic monomer (M3).

Cationic comonomer M1, and optionally M3 will increase solubility anddispersability of the copolymer in an aqueous system; whereas non-ionicmonomer units M2 will reduce water solubility, and promote formingaggregates and activity as emulsion stabilizer or polymeric surfactant,by non-polar or hydrophobic interactions. Too high an amount of M2 mayresult in insolubility and/or precipitation of the copolymer in anaqueous medium. The type and amount of comonomers is thus preferablychosen such that the copolymer can be dispersed in an aqueous mediuminto colloidal particles. It is an advantage of present invention thatrandom copolymers of such monomers can already function as emulsionstabilizers in an aqueous medium; thus omitting the need for using morecomplex synthetic routes of making block copolymers. The skilled personwill be able to select a suitable copolymer composition, also dependingon the compound A to be emulsified and composition of the aqueousmedium.

In a preferred embodiment, the copolymer C used in the method accordingto the invention is a copolymer obtained from

-   -   1-25 mole % of at least one monomer M1;    -   50-99 mole % of at least one monomer M2; and    -   0-25 mole % of at least one monomer M3 (with the sum of M1, M2,        and M3 adding up to 100%).

If a comonomer M3 is used in preparing the copolymer, especially ananionic M3, monomer M1 is used in such amount, for example in a molarexcess over M3, to result in an ionic copolymer having a net positivecharge.

Preferably, the copolymer used in the process according to the inventionis a cationic copolymer obtained from

-   -   5-15 mole % of at least one monomer M1;    -   75-95 mole % of at least one monomer M2; and    -   0-10 mole % of at least one monomer M3.

In further preferred embodiments, the copolymer used in the processaccording to the invention is such a cationic copolymer obtained from atleast 5, 6, 7, 8, 9 or 10 mole %, and at most 25, 20, 15, 12 or 10 mole% of at least one monomer M1; and at least one monomer M2 in such amountthat the sum of M1 and M2 is 100 mole %.

Cationic monomers M1 that can be suitably used in forming copolymer Cused the process according to the invention via addition polymerisationinclude vinyl monomers with a pending amino functional group; which canbe non-ionic monomers that can be neutralised during or after formingthe copolymer, monomers with an already neutralised amino functionalgroup, or vinyl monomers with a permanent quaternary ammonium group.

Examples of vinyl monomers bearing non-ionic amino functional groupsinclude N,N-dimethylaminoethyl (meth)acrylate, N,N-dimethylaminohexyl(meth)acrylate, N,N-diethylaminoethyl (meth)acrylate,N-methyl-N-butyl-aminoethyl (meth)acrylate, tert-butylaminoethyl(meth)acrylate, N,N-dimethylaminopropyl (meth)acrylate,2-(1,1,3,3,-tetramethylbutylamino)ethyl (meth)acrylate,beta-morpholinoethyl (meth)acrylate, 4-(beta-acryloxyethyl) pyridine,vinylbenzylamines, vinylphenylamines, 2-vinylpyridines or4-vinylpyridines, p-aminostyrenes, dialkyaminostyrenes such asN,N,-diaminomethylstyrene, substituted diallylamines,N-vinylpiperidines, N-vinylimidazole, N-vinylimidazoline,N-vinylpyrazole, N-vinylindole, N-substituted (meth)acryl amides like2-(dimethylamino)ethyl (meth)acrylamide, 2-(t-butylamino)ethyl(meth)acrylamide, 3-(dimethylamino)propyl (meth)acrylamide, (meth)acrylamide, N-aminoalkyl (meth)acrylamides, vinyl ethers like 10-aminodecylvinyl ether, 9-aminooctyl vinyl ether, 6-(diethylamino)hexyl vinylether, 5-aminopentyl vinyl ether, 3-aminopropyl vinyl ether,2-aminoethyl vinyl ether, 2-aminobutyl vinyl ether, 4-aminobutyl vinylether, 2-dimethylaminoethyl vinyl ether,N-(3,5,5,-triethylhexyl)aminoethyl vinyl ether, N-cyclohexylaminoethylvinyl ether, N-tert-butylaminoethyl vinyl ether, N-methylaminoethylvinyl ether, N-2-ethylhexylaminoethyl vinyl ether, N-t-octylaminoethylvinyl ether, beta-pyrrolidinoethyl vinyl ether, or(N-beta-hydroxyethyl-N-methyl) aminoethyl vinyl ether. Cyclic ureido orthiourea containing ethylenically unsaturated monomers like(meth)acryloxyethyl ethyleneurea, (meth)acryloxyethyl ethylenethiourea(meth)acrylamide ethyleneurea, (meth)acrylamide ethylenethiourea andalike can also be used. Preferred monomers are amino-functional(meth)acrylates and (meth)acrylamides; especially N,N,-dialkylaminoalkyl(meth)acrylates, more specifically t-butylaminoethyl methacrylate,dimethylaminopropyl methacrylate, dimethylaminoethyl methacrylate(DMAEMA) or diethylaminoethyl methacrylate (DEAEMA), more preferablyDMAEMA and DEAEMA.

The above given examples of suitable and preferred M1 monomers can alsobe used in their ionised form, by treating with for example an acid,preferably an organic acid like a carboxylic acid, prior topolymerisation.

Suitable examples of M1 monomers with a permanent quaternary ammoniumgroup include methacrylamidopropyl trimethylammonium chloride (MAPTAC),diallyl dimethyl ammonium chloride (DADMAC), 2-trimethyl ammonium ethylmethacrylic chloride (TMAEMC) and quaternary ammonium salts ofsubstituted (meth)acrylic and (meth)acrylamido monomers.

Neutral or non-ionic monomers M2 that can be suitably used in formingthe copolymer used the process according to the invention via additionpolymerisation include a wide range of ethylenically unsaturatedmonomers or vinyl monomers, including various styrenic, (meth)acrylic,olefinic, and/or vinylic comonomers. The at least one monomer M1 ispreferably hydrophobic. Preferably, the cationic copolymer comprises acertain amount of non-water soluble or hydrophobic comonomers that willpromote the copolymer, not being fully water soluble, to self-assembleinto colloidal particles or aggregates in an aqueous medium. The skilledperson will be able to select suitable combinations of monomers andtheir contents based on the information disclosed in this descriptionand experiments, possibly assisted by some further experiments; anddepending on copolymer composition (like M1 and M2 types and amounts),conditions (like solvent composition, temperature, pH), and type ofcompound A.

Suitable styrene monomers M2 include styrene, alpha-methyl styrene andother substituted styrenes. Suitable (meth)acrylic monomers M2 includealkyl or cycloalkyl (meth)acrylates, preferably C₁-C₁₈ alkyl(meth)acrylates or C₁-C₈ alkyl (meth)acrylates, like methyl(meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate (allisomers), isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,isopropyl (meth)acrylate, propyl (meth)acrylate (all isomers), orisobornyl (meth)acrylate. Most preferred (meth)acrylic monomers includemethyl methacrylate (MMA), ethyl methacrylate (EMA), n-butylmethacrylate (BMA). Similarly, N-alkyl (meth)acrylamides can be used asmonomer M2. Also other monomers that can be copolymerized with M1 can beused as monomer M2, including butadiene; vinyl monomers like vinylchloride, vinyl laurate, vinyl alkyl ethers, and the like.

As monomer M3 various compounds that can be copolymerized with M1 and M2can be used, including acrylonitrile, methacrylonitrile, vinyl monomerslike vinyl pyrrolidone, vinyl esters such as vinyl acetate or vinylpropionate, and the like. Also anionic or acidic monomers M3 may be usedin forming the copolymer used in the method according to the invention;like monomers with a pending phosphoric, sulfonic, or carboxylic acidgroup. Suitable monomers include vinyl monomers with a carboxylic acidgroup, examples being ethylenically unsaturated monocarboxylic and/ordicarboxylic acids, like fumaric acid, itaconic acid, maleic acid, andespecially (meth)acrylic monomers with a carboxylic acid group, such asacrylic acid (AA), methacrylic acid (MAA) and β-carboxy ethylacrylate. Acopolymer comprising both anionic and cationic groups can be referred toas a polyampholyte. Resulting intra- and intermolecular interactionsbetween the ionic groups may enhance their functioning as emulsionstabilizer in the process of the invention.

In a preferred embodiment of the process according to the invention, thecationic copolymer is obtained from 5-15 mole % of at least one monomerM1 selected from the group consisting of amino-functional(meth)acrylates and (meth)acrylamides; and 85-95 mole % of at least onemonomer M2 selected from the group of C1-C18 alkyl (meth)acrylates.

In a further preferred embodiments of the process according to theinvention, a cationic copolymer is applied which is obtained from 5-10mole % of dimethylaminoethyl methacrylate (DMAEMA) as monomer M1, and90-95 mole % of MMA as monomer M2; or a copolymer made from 5-12 mole %of DMAEMA and 88-95 mole % of isobornyl acrylate (IBOA).

In the process according to the invention the molar mass of copolymer Ccan vary widely. Typically, the copolymer has a weight averaged molarmass (Mw) in the range 1-500 kDa (kg/mol), preferably Mw is at least 2,5, 10, 15 or even 20 kDa, but at most 250, 200, 150, 100, 50 or even 40kDa to optimise activity as emulsion stabilizer. The molar mass of thecopolymer can be determined by gel permeation chromatography (GPC) usingpolymethylmethacrylates of known molar masses as a standard andhexafluoro iso-propanol as a solvent.

In step 1) of the process according to the invention an oil-in-wateremulsion is prepared by mixing compound A and copolymer C in an aqueousmedium of pH 2-6, resulting from organic and/or inorganic acid or buffercompounds being present. Within this pH range the copolymer C will becationically charged, also if M1 monomer has a (neutralized) tertiaryamine pending group. Temperature for mixing is not critical, but isgenerally from ambient to about 50° C.

By aqueous medium is herein meant a liquid comprising water. The aqueousmedium may in addition to water comprise at least one organic solventthat can be dissolved in or is miscible with water, like alcohols,ketones, esters, or ethers. Examples include 1,4-dioxane, acetone,diethyl acetate, propanol, ethanol, methanol, butanol, methyl ethylketone, methyl propyl ketone, and tetrahydrofuran. Preferably, theaqueous medium comprises water and a lower (C1-C8) aliphatic alcohol,like methanol, ethanol, iso-propanol or 1-methoxypropan-2-ol; morepreferably ethanol or iso-propanol. If organic solvent is present, itsamount is chosen such that compound A and copolymer C will not bedissolved, securing formation of emulsified droplets of compound A andcopolymer C. Preferably the amount of organic solvent is at most 10 mass%, more preferably at most 8, 6 or 4 mass % (based on totalcomposition). Preferably the solvent composition of the aqueous mediumis also suitable for the subsequent step 2) of forming a shell layer.

In the process according to the invention the manner and order in whichcomponents A and C are added to and mixed with the aqueous medium is notparticularly critical. For example compound A and copolymer C may beadded simultaneously or sequentially, optionally under stirring.Formation of emulsified droplets may be facilitated by stirring, forexample with a stirring bar at relatively low speed, with a high-speedmixer, with a high-pressure homogenizer, or with sonication. The actualstirring rates (or similar settings when using alternative mixingdevices as exemplified above) can vary and is chosen to realize anoil-in-water emulsion as determined by routine experimentation by theskilled person.

In the step of preparing an oil-in-water emulsion by mixing organiccompound A and copolymer C in aqueous medium the mass ratio of C to A isfrom 0.1 to 2. A certain minimum amount of cationic copolymer is neededto stabilize the emulsion, but also to provide the emulsified dropletswith a certain charge level relevant for forming a shell layer of atleast partially reacted inorganic oxide precursor in subsequent step 2).Too high an amount of copolymer, that is a relatively low amount ofcompound A, could reduce potential porosity level that can be formed ina coating layer from the composition as obtained. Mass ratio C/A istherefore preferably at least 0.15, 0.2, 0.3 or 0.4, and at most 1.5,1.2, 1.0, 0.9, 0.8, 0.7, 0.6 or 0.5.

If desired, in the process according to the invention a furtherconventional surfactant of low molar mass may be added in step 1), toassist formation of dispersed droplets and to further stabilise theemulsion obtained. The surfactant used may be non-ionic, cationic oranionic, or a combination thereof. Preferably none, or only small amountof surfactant is used, like 0.1-3, more preferably 0.2-1.5 mass %.

The process of the invention results in step 1) in a dispersion of 1-50mass % of emulsified droplets (based on total emulsion). The emulsionmay be prepared with a wide range of concentration of emulsifiedparticles, depending subsequent steps and uses. Generally a relativelyhigh concentration is preferred in step 1), allowing further dilutionwith water or other solvents as required during subsequent steps.Preferably, a dispersion is made in step 1) containing at least 2, 4, 6,8 or 10 mass % of emulsified droplets, and at most 40, 30, 25 or 20 mass%. Based on the information provided above, the skilled person candetermine the relative amounts of compound A, copolymer C, aqueousmedium and optionally other compounds to be used in the process.

The process of the invention results in step 1) in a dispersion ofemulsified droplets of particle size 30-300 nm. Formation and size ofemulsified droplets can be monitored by various techniques; for exampleby Dynamic Light Scattering (DLS). In view of using the compositionobtained with the process according to the invention to make an ARcoating, variables of the process as described above are chosen suchthat particle size is preferably at least 35, 40, 45 or 50 nm, and atmost 250, 200, 175, 150 or 125 nm (as measured with DLS).

The process of the invention comprises a step 2) of providing aninorganic oxide shell layer to the emulsified droplets, by adding to theemulsion obtained in step 1) at least one inorganic oxide precursor toresult in organic/inorganic core-shell nano-particles with mass ratiocore/shell of from 0.2 to 25. The core is here the sum of compound A andemulsion stabilizer C and shell is SiO₂ equivalent of inorganic oxideprecursor. By metal oxide equivalent of inorganic oxide precursor ishere meant the mass of oxides that the precursors is converted to bycomplete conversion into metal oxides, such as TMOS and TEOS eachcounting as one SiO₂, titanium tetraisopropoxide counting as one TiO₂and aluminium nitrate counting as a half Al₂O₃. In the step 2) of theprocess according to the invention, the emulsified droplets act astemplate on which a shell layer is formed from sol particles ofpartially reacted precursor. Such formation of an inorganic shell layerby a sol-gel process has been described in many publications, includingdocuments cited in this application, and publications referencedtherein. Suitable inorganic oxide precursors include metal salts, metalchelates and organo-metallic compounds, preferably metal alkoxides, andcombinations thereof. Such compounds can undergo various hydrolysisand/or condensation reactions in aqueous medium to finally formcorresponding oxides or mixed oxides. Within the present applicationsilicon (Si) is considered to be a metal. Suitable metals include Si,Al, Bi, B, In, Ge, Hf, La and lanthanoids, Sb, Sn, Ti, Ta, Nb, Y, Zn andZr, and mixtures thereof. Preferably the metal is at least one of Si,Al, Ti, and Zr. Preferred precursors include alkoxy silanes, preferablytetra- or tri-alkoxy silanes like tetramethoxy silane (TMOS),tetraethoxy silane (TEOS), methyltrimethoxy silane, methyltriethoxysilane, titanium tetraisopropoxide, aluminium nitrate, aluminiumbutoxide, yttrium nitrate and zirconium butoxide. Such precursorcompounds can be partially pre-reacted or pre-hydrolysed to formoligomeric species, typically in the form of nano-sized particles ofabout 1-20, 1-10 or even 1-5 nm; also called sol particles.

In a preferred embodiment of the invention, the at least one inorganicprecursor comprises an alkoxy silane, more preferably TMOS and/or TEOS.Preferably, the shell layer formed substantially consists of silica(precursor) as inorganic oxide; the shell for example comprises at least80, 85, or 90 mole % of Si as metal in the inorganic oxide, morepreferably at least 95 mole % of Si.

In the process according to the invention the step of forming a shelllayer from the precursor on the template to result in core-shellnano-particles is typically performed under mild conditions. Asmentioned above, the aqueous medium may comprise an organic solvent thatis miscible with water, like alcohols, ketones, esters, or ethers;preferably an alcohol like methanol, ethanol or iso-propanol. Waterserves as solvent or diluent for the composition, but will also reactwith the inorganic oxide precursor; for example with an alkoxy silane.The amount of water present in the composition is therefore at least theamount needed for such desired reaction(s), like (partial) hydrolysis offor example tetraethoxy silane. In case complete hydrolysis of TEOSwould be aimed at, the composition should contain water in at least a4:1 molar ratio to Si.

The temperature is not very critical in this step of the process of theinvention and can be varied widely as long as the emulsion is notdisrupted. Temperature can be up to 100° C., but is typically ambient,i.e. from about 15 to 40° C. As said hydrolysis reaction is exothermic,cooling may be used to control temperature in this step. The pH is inthe range 2-6, preferably 3-5 or 3-4.5. An advantage of applying suchconditions is that nanoparticles formed from the precursor typicallyhaving a negative charge and will at least partly deposit on the outsideof emulsified droplets of opposite charge. This way an open or ‘fluffy’,or more condensed layer of inorganic oxide (precursor) may form aroundthe emulsion particles, depending on reaction conditions.

In the process according to the invention forming a shell layer from theprecursor on the template droplets is performed at such mass ratio ofinorganic oxide precursor to organic template that organic/inorganiccore-shell nano-particles with mass ratio core/shell of from 0.2 to 25result. It is preferred that the mass ratio core/shell is from 0.2 to 5and particularly from 0.2 to 2. More preferably, the process results incore-shell nano-particles with mass ratio core/shell of 0.3-2, morepreferably 0.4-1.8. The high mass ratio of core/shell such as 2 to 25and particularly from 4 to 23 is particularly advantageous when noseparate binder addition step 3) is included in the method according tothe invention. In this case, oxide precursor added in step 2) may alsoact as an integrated binder, in the sense that some of the oxideprecursor will form the shell of the core-shell nano particles and someof the oxide precursor will remain unbonded or only very loosely bondedto the nano particles and during preparation of the ARC will act as abinder.

Formation of a shell layer from the precursor on the template may bemonitored by measuring change in dimensions of the emulsified droplets,eg. by DLS. Although the DLS technique has its draw-backs, for examplemainly detecting the larger particles, and particle size may also changeas result of compounds liberated from the precursor, like an alcohol,which could be absorbed in the core, it is a simple and convenientmethod to observe shell formation. Shell formation may slow down or stopwhen the net charge of the emulsified core particle has decreased by theinorganic oxide (precursor) having charge opposite to that of thecopolymer. A certain charge level will likely remain to keep theparticles dispersed. As shell formation is thought to result fromcomplexing of inorganic nanoparticles with the outer layer of emulsifieddroplet comprising cationic copolymer, an open or fluffy structure isexpected to be formed in the aqueous medium rather than a dense shell(as in dried particles).

In embodiments of the process according to the invention the structureof the shell formed, like its density or surface properties, may befurther optimized by extending reaction time, reacting with a couplingagent or other treatment as known from the art. Thickness of the shelllayer thus formed is typically in the range of 1-20 nm, preferably 2-10nm. Shell thickness of core-shell nano-particles, and their morphologycan be assessed on particles with techniques like TEM, especiallycryo-TEM, SAXS, or SANS. Considering the relatively thin shell layer,particle size of the core-shell particles is in ranges similar to thoseof the emulsified droplets.

With the process according to the invention dispersed core-shellnano-particles are obtained, which composition is found to showremarkably good storage and handling stability, meaning the compositionshows little tendency to changing viscosity or gelling compared to othersol-gel process based dispersions. It was further found that the solidscontent of the dispersion may be adjusted by evaporation or dilutionwith water or organic solvent like an alcohol; which greatly increasesthe possibilities for adjusting composition to match requirements forcoating applications.

The process according to the invention preferably comprises a furtherstep 3) of adding 2-70 mass % of at least one polymeric or polymerisablecompound as binder (mass % based on the sum of core-shell particles andbinder). In principle the composition obtained after step 2) can be usedto form an ARC on a substrate, showing a certain level of adhesion tothe surface of the substrate after drying and optionally curing,resulting from further reaction of the inorganic oxide precursor, whichgenerally has not fully reacted during preparing the shell layer.Preferably, a binder is added in step 3) to the AR coating composition,which binder can be at least one inorganic or organic polymeric orpolymerisable compound. In forming an ARC from the composition, thebinder may act as film former and hold together the core-shellnano-particles, resulting in improved mechanical properties of thecoating formed and in better adhesion to a substrate upon drying and/orcuring. Addition of binder will reduce the level of porosity of acoating made from the composition. Thus in step 3) of the processpreferably at least 2, 5, 10, 15, 20, or 25 mass % of binder is added;but at most 65, 55, 50, 40, or 30 mass % (based on the sum of core-shellparticles and binder) of binder is used.

Suitable organic binders in the process according to the inventioninclude a range of different polymers and thermally or radiation—e.g.UV—curable monomers, as known to the skilled person. Organic binders,especially radiation curable binders generally have the advantage thatthey can be cured at relatively low temperatures of preferably below250° C., compatible with e.g. thermoplastic substrates, and at whichalso the organic compound A may be evaporated from the nano-particles toin situ create hollow particles. Use of inorganic binders may bepreferred to result in coatings with improved mechanical properties anddurability, but often requiring curing at elevated temperatures of about250-900° C. It is an advantage of present invention that the processallows the skilled person to select and use a binder that provides theproperties desired for a certain application of the coating. Examples ofsuitable organic binders that may be applied include radicalcurable—peroxide- or photo-initiated—compositions comprising vinylmonomers and vinylpolymers having unsaturated groups, like acrylates,methacrylates, maleate/vinyl ethers, etc.), or radical curableunsaturated polyesters or polyurethanes in styrene and/or(meth)acrylates.

Suitable inorganic binders in the process of making an AR coatingcomposition of the invention include inorganic oxide precursors likemetal alkoxides, metal chelates, metal salts, and mixtures thereof.Suitable metals include at least one element selected from Si, Al, Be,Bi, B, Fe, Mg, Na, K, In, Ge, Hf, La and lanthanoids, Sb, Sn, Ti, Ta,Nb, Y, Zn and Zr; preferably the metal is at least one element selectedfrom Si, Al, Ti, and Zr. Suitable inorganic oxide precursors includethose compounds that can react via hydrolysis and/or condensationreactions to form the corresponding oxide, as is well known in the art.The inorganic oxide precursor (which in the context of the presentinvention is considered polymerisable through for example hydrolysisand/or condensation, or polymerized in a glass, sol-gel or crystalstage) can be a metal salt or an organo-metallic compound, like analkoxy, an aryloxy, a halogenide, a nitrate, or a sulphate compound, andcombinations thereof. Preferred precursors include alkoxy silanes,including halogenated—especially fluorinated—derivates, liketetramethoxy silane (TMOS), tetraethoxy silane (TEOS), methyltrimethoxysilane, methyltriethoxy silane, fluoroalkoxy silanes liketrifluoropropyl trimethoxy silane, titanium tetraisopropoxide, aluminiumnitrate, aluminium butoxide, yttrium nitrate and zirconium butoxide.More preferably, the precursor comprises TMOS and/or TEOS.

The inorganic oxide precursor can be a mixture of inorganic oxideprecursor compound and corresponding inorganic oxide. Such mixture mayfor example result in case a precursor compound partially pre-reacts orpre-hydrolyses in aqueous medium during making the composition to formoligomeric species, typically in the form of nano-sized particles; whichis a well-known procedure in sol-gel technology.

In a further preferred embodiment, the binder used in the process of theinvention comprises a mixture of different inorganic oxide precursors,in which case typically a mixed inorganic oxide is formed as is knownfor e.g. different glasses. In such mixed oxide the elements areconnected via oxygen atoms to form part of an ionic or covalent network,rather than that they are present as a physical mixture of differentoxides. Within the context of the present disclosure, mixed inorganicoxide refers to such definition. Formation of a mixed oxide may e.g. bedetermined by assessing changes in iso-electric point of oxides—e.g. inthe form of thin layers—formed from different compositions, or byanalytical techniques, like IR and solid-state NMR. Nevertheless, it iscustomary in the art to define the composition of such mixed inorganicoxide by the theoretical amounts of inorganic oxide for each metalpresent; e.g. the composition of an aluminosilicate made from Si- andAl-oxide precursors is typically expressed in silica and aluminacontents. In case of a mixed oxide as binder, a main metal element ispreferably selected from Si, Al, Ti, and Zr, and a second elementselected from Si, Al, Be, Bi, B, Fe, Mg, Na, K, In, Ge, Hf, La andlanthanoids, Sb, Sn, Ti, Ta, Nb, Y, Zn and Zr; with a molar ratio ofmain to second element of about 75:25 to 99:1.

Preferably, the binder used in step 3) of the process comprises amixture of a silica precursor and a precursor for Al-oxide or Y-oxide,as the mixed oxide formed shows high outdoor resistance or durability.

The process according to the invention may also comprise adding acombination of inorganic and organic binders, to for example furtherimprove properties of the resulting coating, like anti-foulingbehaviour, or enhance adhesion to the substrate. These binders may formpolymers or networks on their own, but can also co-react.

In a preferred embodiment, the binder used in the process according tothe invention consists of at least one inorganic oxide precursor.

The process according to the invention may optionally comprise a furtherstep of adding at least one auxiliary component, which typically is anon-volatile or solid component. Preferably, auxiliary components areadded in an amount of less than 20 mass % based on the sum of core-shellparticles and binder, more preferably less than 10 or 5 mass %. Thesecomponents may be added to assist in processing of the coatingcomposition or to affect other functionalities of the coating to be madefrom the composition. Examples of auxiliary components include acids,buffer agents, catalysts, coupling agents, surfactants, anti-foamingagents, chelating agents, slip agents, thickening agents, and levelingagents.

The process according to the invention optionally comprises a furtherstep of adjusting the solids content of the coating composition byremoving or adding water and/or organic solvent. The AR coatingcomposition made with the process of the invention typically has solidscontent of less than about 20, 15, 10, 5 or even 3 mass %, and a minimumsolids content of about 0.1 mass %, preferably at least 0.2, 0.5 or 1.0mass %. Within the context of this application solids content means thetotal of components added excluding compound A, water and organicsolvents, that is the sum of copolymer C, inorganic oxide precursor,binder and auxiliary components.

The composition made with the process of the invention comprises waterand organic solvent as defined above, water and organic solvent aretogether also referred to as solvent. The solvent of the coatingcomposition obtained with the process is a liquid component thatcontains the other coating components in dissolved, dispersed orcolloidal states, and could thus also be referred to as diluent. Theamount of solvent can be varied to obtain a desired viscosity of thecoating composition, which viscosity may be relatively low to allow easyapplication to a substrate in thin films, e.g. for use as AR coating.Typically the viscosity of the coating composition is at least about 0.6mPa·s, preferably at least 1.0 or 2.0 mPa·s. Depending on the depositiontechnology applied, the viscosity may as high as 1000 mPa·s. Preferablyviscosity is at most 500, 300 or 200 mPa·s. for making thin layers ofhomogeneous thickness. The viscosity can be measured with known methods,for example with an Ubbelohde PSL ASTM IP no 1 (type 27042) especiallyfor low viscosity ranges, or with a Brookfield viscometer. Solidscontent may be adjusted by removing solvent by e.g. evaporation or byadding solvent.

In a further preferred aspect of the process according to the inventionthe pH of the coating composition obtained is changed to a level atwhich inorganic oxide and/or its precursor—present in core-shellparticles and/or as binder—will not react, including reacting at leastonly very slowly, to prevent agglomeration of core-shell particles andpremature curing of binder; in case of silica precursors preferably to apH of about 2-3, or even below 2 (as measured with a standard pHelectrode on aqueous or alcoholic dispersion). This way the processresults in a composition with favourable storage properties and extendedshelf-life. For adjusting pH an inorganic or organic acid may be added,like nitric acid solution.

Storage at temperature of below room temperature, more preferably below15 or 10° C. but above freezing temperature, will also increaseshelf-life of the coating composition obtained.

The above described steps of the process according to the invention aretypically performed at ambient pressure, but the skilled person willrealise that increased (or reduced) pressure may also be applied.

The invention further relates to an AR coating composition obtained withthe process according to the invention as described hereinabove,including all combinations and perturbations of indicated steps,components, features and embodiments.

In a further aspect the invention also relates to a process for makingan ARC on a transparent substrate comprising the steps of

-   -   applying the coating composition according to the invention or        obtained with the process according to the invention to the        substrate; and    -   drying and curing the applied coating layer.

The transparent substrate on which the coating composition according tothe invention can be applied may vary widely, and can be organic orinorganic and of various geometries. Preferably, the substrate istransparent for at least visible light. Suitable substrates includeinorganic glasses (e.g. borosilicate glass, soda lime glass, glassceramic, aluminosilicate glass) and plastics (e.g. PET, PC, TAC, PMMA,PE, PP, PVC and PS) or composite materials like laminates. Preferablythe substrate is a glass, like borosilicate glass; preferably a flatglass like float glass with smooth or patterned surface.

The coating composition of the invention can be applied directly to thesubstrate, but also to another coating layer already present on thesubstrate; like a barrier layer for alkali ions, an adhesion promotinglayer, a hard coat layer, or a layer having a higher refractive index(than the substrate).

The process according to the invention may also apply more than onecoating layer, preferably with intermediate drying performed after theapplication of each layer. In some embodiments, intermediate drying andcuring is performed after applying some or all of the layers.

In the process according to the invention the AR coating composition canbe applied to the substrate with various deposition techniques, as knownto a skilled person for making thin homogeneous coating layers. Suitablemethods include spin-coating, dip-coating, spray-coating, roll-coating,slot die-coating, aerosol coating and the like. Preferred methods aredip-coating, roll-coating and slot die-coating. The thickness of the wetcoating layer to be applied depends on the amount of solid film formingcomponents in the coating composition, and on the desired layerthickness after subsequent drying and curing. The skilled person will beable to select appropriate methods and conditions depending on thesituation.

The coating composition is preferably applied to the substrate formaking a single layer AR coating in such wet thickness that will resultin a thickness after drying and/or curing of about 20 nm or more,preferably the applied cured coating has a layer thickness of at leastabout 50 or 70 nm and of at most about 200, 180, 160 or 140 nm. In caseof a multi-layer coating the skilled person may select different layerthicknesses.

In the coating process according to the invention the step of drying andcuring the applied coating composition will comprise drying to evaporateat least part of the solvent and other volatile components includingcompound A, and then curing to complete reaction of the binder into forexample inorganic oxide(s), and removing residual volatiles andoptionally non-volatile organic components like the copolymer; dependingon curing temperature.

Drying preferably takes place under ambient conditions (e.g. 15-30° C.),although elevated temperatures (e.g. up to about 250° C., morepreferably up to 100, 50 or 40° C.) may also be used to shorten thetotal drying time. Drying may be promoted by applying an inert gas flow,or reducing pressure. Specific drying conditions may be determined by aperson skilled in the art based on components to be evaporated.

During drying also compound A and/or organic solvent contained in thedispersed core-shell particles may at least partly be removed; resultingin porous or hollow particles. It is a specific advantage of theinvention that an AR coating can be made at relatively low temperature,allowing use of substrates with limited thermal resistance, like plasticsubstrates. In such way of performing the process of the invention, alsothe curing step is performed at a temperature compatible with thesubstrate. After curing a substrate provided with a porous coating andshowing AR properties is thus obtained.

Preferably the applied layer is cured after drying, i.e. aftersubstantially removing volatiles. Curing may be performed using a numberof techniques including thermal curing, flash heating, UV curing,electron beam curing, laser-induced curing, gamma radiation curing,plasma curing, microwave curing or combinations thereof. Curingconditions are depending on the coating composition and curing mechanismof the binder, and on the type of substrate. The skilled person is ableto select proper techniques and conditions. Thermally curing coatings ate.g. temperatures above 250° C. is preferred for inorganic oxideprecursors as binder to result in e.g. better mechanical properties.Such conditions are often not possible for curing a plastic substrate inan oven; and also not needed to generate a desired porosity level withthe AR coating composition of the invention. If high temperature curingis desired, a surface heating technique like flash heating mayadvantageously be applied to minimise exposure of the substrate to hightemperature; as is for example described in WO2012037234.

In a preferred way of operating the process of the invention, curing isperformed at a temperature of at most 300° C., more preferably at most250, or 200° C. After curing the coating, residual organics includingcopolymer C can be optionally further removed by known methods; forexample by exposing the coating to a solvent and extracting the organiccompound from the coating.

Alternatively, especially in case of inorganic binder and glasssubstrate, curing may be performed by heating at temperatures from about250 to 900° C., preferably above 300, 400, 450, 500, 550 or 600° C.,during at least several minutes. Such heating above the decompositiontemperature of organic compound or polymer will remove such compounds toresult in porosity, and also promote formation of oxides from inorganicoxide precursors, especially when in the presence of oxygen; resultingin both curing and removing organics by calcination. In case of aninorganic glass substrate curing can be performed at relatively hightemperatures; of up to the softening temperature of the glass. Suchcuring by heating is preferably performed in the presence of air, and isoften referred to as firing in e.g. glass industry. If desired, the airmay comprise increased amounts of water (steam) to further enhancecuring and formation of an inorganic oxide coating. The product obtainedby such process is typically a fully inorganic porous coating.

In a further preferred embodiment of the coating process of theinvention such curing step is combined with a glass tempering step; i.e.heating the coated glass substrate to about 600-700° C. during a fewminutes, followed by quenching, to result in AR-coated toughened orsafety glass.

The invention further relates to an AR coated transparent substrate thatis obtainable by or is obtained with the process according to theinvention and as described hereinabove, including all combinations andperturbations of indicated features and embodiments.

An anti-reflective (AR) or light reflection reducing coating is acoating that reduces the reflection of light from the surface of asubstrate at one or more wavelengths between 425 and 675 nm, as measuredat 5° incident angle. Measurements are carried out on the coated anduncoated substrate. Preferably the reduction in reflection is about 30%or more, preferably about 50% or more, more preferably about 70% ormore, even more preferably about 85% or more. The reduction inreflection as expressed in a percentage is equal to 100×(reflection ofthe uncoated substrate−the reflection of the coatedsubstrate)/(reflection of uncoated substrate).

Typically, the AR coated substrate obtainable by the process accordingto the invention shows very good AR properties, in combination with goodmechanical performance, like abrasion resistance passing the felt testas defined in EN1096-2. The AR coated substrate according to theinvention shows at a coated side a minimum reflection of 2% or less at acertain wavelength, preferably about 1% or less, and more preferably ofat most about 1.4, 1.2, 1.0, 0.8 or 0.6% (for two-sided coatedsubstrate). The average reflection over a 425-675 nm wavelength rangefor a two-sided coated substrate is preferably about 3% or less, andmore preferably at most about 2.5, 2, 1.8, 1.7, 1.6 or 1.5%.

The AR coated substrate according to the invention may be used in manydifferent applications and end-uses, like window glazing, cover glassfor solar modules, including thermal and photo-voltaic solar systems, orcover glass for TV screens and displays. The invention thus furtherrelates to an article comprising the AR coated substrate obtained withthe process according to the invention. Examples of such articlesinclude solar panels, like a thermal solar panel or a photo-voltaicmodule, monitors, touch-screen displays for mobile phones, tablet pc'sor all-in-one pc's, and TV sets.

The invention further relates to a method of making organic-inorganiccore-shell or inorganic hollow nano-particles. More specifically, theinvention concerns a method for making organic-inorganic core-shellparticles comprising steps 1) and 2) of the process of making ananti-reflective coating composition as described in the above, includingall preferred embodiments. The method may further comprise a step ofisolating the nano-particles from the dispersion obtained; using anymethod as known in the art like filtration, freeze-drying orspray-drying techniques.

The invention also relates to a method of making hollow inorganicnano-particles comprising steps 1) and 2) of the process of making ananti-reflective coating composition as described in the above and afurther step of at least partially removing the core or template fromthe core-shell nano-particles to result in a porous or hollow core (forsimplicity together referred to as hollow core). The organic core maycomprise compound A, organic solvent, copolymer C, and optionally othersurfactant. This core can be at least partly removed by evaporatingvolatile components; and/or by solvent extraction or etching, thermaldegradation, catalytic decomposition, photo-degradation, electron beamor laser irradiation, and combinations thereof; optionally followed byevaporating the degradation products. The method may further comprise astep of isolating the nano-particles from the dispersion obtained; usingany method as known in the art like filtration, freeze-drying orspray-drying techniques. Core material may be removed, partially orvirtually completely, while the nano-particles are still in dispersedform, but also during or after separating the particles from thedispersion for a subsequent use

The invention further relates to organic-inorganic core-shell orinorganic hollow nano-particles as obtained with said methods of theinvention, to compositions comprising such nano-particles, and todifferent uses of said nano-particles and compositions. Productsobtained with the methods according to the invention are hybridorganic-inorganic particles or hollow inorganic nano-particles, indispersed form or as dried nano-particles. Dispersed product is found toshow remarkably good storage and handling stability, meaning thedispersion shows little tendency to changing viscosity or gellingcompared to other sol-gel process based dispersions. It was furtherfound that the solids content of the dispersion may be adjusted byevaporation or dilution, which greatly increases the possibilities forusing the dispersion obtained in a number of different applications.

The nano-particles made with the method according to the inventiontypically have an particle size of at most 300 nm, more preferably atmost 250, 200, 150, or 100 nm. Preferably, the particle size ispreferably at least 35, 40, 45 or 50 nm. The size and shape of theindividual core-shell nano particles varies considerably for nanoparticles of a coating composition according to the invention. It istherefore emphasized that particle size herein refer to the Z-averagedhydrodynamic diameter measured by determined by Dynamic Light Scattering(DLS) on dispersions on a Malvern Nano ZS as discussed above.

The invention further relates to compositions comprising thenano-particles as obtained with the methods according to the invention;and to different uses of said nano-particles and compositions, as wellas to products comprising or made from said nano-particles andcompositions, including paint compositions, cosmetic compositions,controlled-release medicaments, and composite materials.

Throughout the description and claims of this specification, the words“comprise” and “contain” and variations of the words, for example“comprising” and “comprises”, means “including but not limited to”, andis not intended to (and does not) exclude other moieties, additives,components, integers or steps.

Throughout the description and claims of this specification, thesingular encompasses the plural unless the context otherwise requires.In particular, where the indefinite article is used, the specificationis to be understood as contemplating plurality as well as singularity,unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties orgroups described in conjunction with a particular or preferred aspect,embodiment or example of the invention are to be understood to beapplicable to any other aspect, embodiment or example described hereinunless stated otherwise or obviously incompatible therewith.

The invention will be further illustrated by the following examples andcomparative experiments, without being limited thereto.

EXPERIMENTS Organic Compounds

Table 1 provides relevant data on compounds A that are applied inexperiments as organic core or template.

TABLE 1 Melting point Boiling point Solubility in Reference Compound (°C.) (° C.) water (kg/m³) A1 Cyclohexane 6.5 81 0.04 A2 Toluene −93 1100.5 A3 Isoamyl −78 142 1.1 acetate

Cationic Copolymers

Table 2 presents monomer composition for a number of cationic copolymersC, which are obtained following the procedure described in theexperimental part of EP2178927, which is incorporated herein byreference. Copolymers were used as aqueous dispersion with aconcentration of about 20 mass %, with pH of about 4 (acidified withformic acid). The copolymers had Mw in the range 25-40 kDa (GPC).

TABLE 2 C1 C2 C3 C4 C5 monomer Comonomer content (mole %) DMAEMA 10.138.3 17.5 10.3 8.3 MMA 89.9 61.7 82.5 — 91.7 IBOA — — — 89.7

DLS Measurements

A Malvern Nano ZS dynamic light scattering instrument was used tomeasure particle size of dispersed particles on 1 drop of dispersion in10 ml aqueous KCl solution (1 mmol/L) at 25° C. and in back-scatteringmode. Particle size herein refers to average particle size measured asZ-averaged hydrodynamic diameter.

Felt Test

Scratch resistance of applied coating layers was evaluated by the felttest according to EN1096-2.

Optical Properties

Reflection and transmission of coated transparent substrates wereevaluated with a Shimadzu UV-2450 spectrophotometer. Relative specularreflectance was measured at an incident angle of 5° with a reflectanceattachment. For measuring transmission the integrating sphere attachmentwas installed in the sample compartment, and incidence angle was 0°(normal to the sample surface).

Average reflection values are calculated for the wavelength range425-675 nm. Measurements are performed on two-sided coated plates.

EXAMPLE 1

20 grams of cyclohexane (p.a.) containing 1 mass % of heptadecane wasdispersed using a Ultra-turrax unit T25 into a mixture of 14 gramsMilli-Q water, 1 gram of 2-propanol and 15 grams of a dispersioncontaining 21.5 mass % of cationic copolymer C1 (Mw about 30 kDa). Theresulting coarse emulsion was further dispersed using a high-pressurehomogenizer (DeBee, operated at a pressure 30 kPsi, using diamondorifice and applying water cooling) in 9 cycles of about 15 strokes eachand allowing the temperature to decrease after each cycle to 40° C. Thisresulted in a stable emulsion with emulsion droplets of particle size(DLS Z-averaged hydrodynamic diameter) of 265 nm (Polydispersity Index,PDI 0.28). To this emulsion 2 grams of copolymer C1 were added,rendering a clear positive charge of the droplets, indicated by the zetapotential >+11 mV (pH 4). Silication was then performed by graduallyadding (90 minutes) by perfusor pump 41.5 g of tetramethoxy silane(TMOS) to a mixture of 35 g of the resulting emulsion and 80 g Milli-Qwater, under firmly stirring with magnetic stirring bar. After theaddition was completed stirring was continued for another 90 minutes.100 g of the mixture was diluted with 100 g of Milli-Q water andacidified with 6 drops of concentrated HNO₃. The DLS size of the finalproduct was 255 nm (PDI 0.20) and TEM analysis revealed spherical silicaparticles with particle size in the range 60-120 nm. (see FIG. 1).

Cyclohexane could be removed by rota-evaporation treatment, whilegradually increasing the water temperature of the water bath from 30 to40° C., and reducing the pressure from 300 to 100 mbar. The finaldispersion contained 6.4 mass % of hollow silica particles with DLS sizeof 219 nm (PDI 0.35) and zeta potential of 12 mV (pH =4); and was foundto be stable in time.

EXAMPLE 2

Example 1 was repeated, but now 20.7 g of TMOS was used. Afterevaporation of cyclohexane TEM analysis showed hollow silica spheres ofsimilar size as in Example 1, but the particles appeared to be partlycollapsed; likely due to limited strength of the silica shell duringsample preparation for the TEM analysis.

Comparative Experiment 3

Example 1 was repeated, but now copolymer C2 was used. It appeared notpossible to obtain a stable emulsion of cyclohexane; polymer C2apparently being too hydrophilic to function as emulsion stabilizer.

EXAMPLE 4

Example 1 was repeated, but now copolymer C3 was used. After evaporationof cyclohexane TEM analysis showed hollow silica particles of similarsize as in Example 1, but the particles appeared to be less regularlyshaped.

EXAMPLE 5

21 g of cyclohexane containing 1 mass % of heptadecane was dispersedusing a Ultra-turrax unit T25 into 51.3 g of a dispersion containing17.4 mass % of cationic copolymer C4 (Mw about 31 kDa). This resulted ina stable emulsion (>1 week) with emulsion droplets of DLS size of 202 nm(PDI 0.02). Silication was performed by gradually adding (90 min) 41.5 gof TMOS to a mixture of 35 g of the resulting emulsion and 80 g Milli-Qwater, while firmly stirring with magnetic stirring bar. After theaddition was completed the dispersion was stirred for another 90minutes. 135 g of the mixture was diluted in 750 g Milli-Q water andacidified with 15 drops of concentrated HNO₃. The final dispersion of2.2 mass % displayed a DLS size of 200 nm (PDI 0.06), a zeta potentialof +19 mV at pH 4.0 and was stable for more than one week. A TEMmicrograph on the resulting dispersion after sample preparation showedspherical hollow silica particles of about 100-150 nm and a shellthickness of about 10 nm; also showing some collapsed particles.

Cyclohexane was removed from the dispersion by spray-drying (Büchi MiniSpray drying B-191) at an evaporation temperature of 130-150° C., flowrate at 270 mL/h in combination with an air flow of 640 normal L/h. TEMperformed on the obtained white powder showed aggregated particleshaving multi-hollow structure. The product showed opacifier (whitening)power, when applied as a simple paint formulation on black photo paper.

EXAMPLE 6

23.3 g of toluene was mixed with 52.6 g of a dispersion of copolymer C5(19 mass % in water, pH 3.9, particle size 44 nm (PDI 0.06) by DLS)using a Dispermat mixing unit, and then diluted with 180 grams of water;resulting in about 13 mass % emulsified droplets in water. To 100 g ofthis emulsion 52 g of TMOS was added drop-wise over 2 hours at ambienttemperature under stirring. Particle size of resulting particles wasabout 82 nm (according to DLS). The obtained dispersion was acidifiedwith 50% nitric acid to a pH of 1.8; and showed stability over time. ATEM micrograph revealed spherical particles showing a core-shellstructure, and particle size in the range 30-80 nm (see FIG. 3).

EXAMPLE 7

50 g of isoamyl acetate was mixed with 113 g of a dispersion ofcopolymer C5 (19 mass % in water, pH 3.9, particle size 44 nm (PDI 0.06)by DLS) using a Dispermat mixing unit, and then diluted with 385 gramsof water; resulting in about 13 mass % emulsified droplets in water. To180 g of this emulsion 70 g of TMOS was added drop-wise over 2 hours atambient temperature under stirring. Particle size of resulting particleswas about 100 nm (according to DLS). The obtained dispersion wasacidified with 50% nitric acid to a pH of 1.9; and showed stability overtime. Moreover, the dispersion could be diluted with water, ethanol, orisopropanol and remain stable (no visual flocculation or sedimentation).

A Cryo-TEM micrograph revealed spherical particles showing a core-shellstructure, and particle size in the range 30-70 nm (see FIG. 4).

Several AR coatings were prepared from the dispersion obtained, bydiluting with isopropanol and adding different amounts of a sol madefrom tetraethoxy silane (TEOS) as binder.

The sol of TEOS was prepared by adding to a solution of TEOS iniso-propanol a molar excess of water while stirring, to pre-hydrolysethe silane compound. After cooling back to room temperature glacialacetic acid was added, and after 24 hrs stirring at ambient conditionsmore iso-propanal and nitric acid (65%) were added. The resultingdispersion contained about 4 mass % of silica particles of about 3-5 nmsize.

Composition 7-1 was made by diluting the core-shell particles dispersionwith 5-fold amount of acidified isopropanol, followed by adjusting pH ofthe final dispersion to 1.5 by adding nitric acid (50%).

Compositions 7-2, 7-3, 7-4 and 7-5 were prepared by mixing an amount ofabove prepared core-shell particles dispersion with different amounts ofthe TEOS sol as binder and iso-propanol, after which the pH was adjustedto about 1.5 by adding nitric acid (50%). The amount of binder wascalculated as mass of SiO₂ resulting from TEOS relative to the sum ofbinder and core-shell particles.

The obtained coating compositions were used to provide coating layers toglass plates by dip-coating in a dust-free room. Pilkington Optiwhite Sglass plates of 2 mm thickness were cleaned with water and householdcleaner, then rinsed with water and demi-water, and then dip-coated byimmersing in a container with coating composition; the coating bathbeing kept at room temperature (at about 21° C.) and 50% relativehumidity. The plate was then vertically pulled up from the bath at arate of about 2.5 mm/s. The coated plate was subsequently dried atambient conditions for at least 5 minutes.

After drying the coated glass was cured at 125° C. during 15 minutes inan air circulation oven, or cured at 650° C. during 2.5 minutes. Allsamples passed the felt test. Minimum reflection of the samples cured at650° C. was between 0.5 and 1%. As an example the reflection curvemeasured for sample 7-1 is shown in FIG. 2. In case the coated plateswere cured at 125° C. the average reflection was between 1.7 and 3.2%.This difference in reflection can be attributed to different porositylevels, resulting from the copolymer present in the core-shell particlesbeing pyrolysed and evaporated at 650° C.; but not at the low curingtemperature, whereas curing at 125° C. will result in evaporation of theorganic compound contained in the core-shell particles dispersion, andthus in some porosity. Reflection data are summarized in Table 3.

Comparative Experiment 8

52.6 g of dispersion of copolymer C5 (19 mass % in water, pH 3.9,particle size 44 nm (PDI 0.06) by DLS) was mixed with 173.3 g of water(˜5% emulsion stabilizer in water), and subsequently 20 g of TMOS wasdrop-wise added over 2 hours at ambient temperature. Increase ofparticle size to about 84 nm (according to DLS) was observed. Theobtained dispersion was acidified with 50% nitric acid to a pH of 1.8;and showed stability over time. A cryo-TEM micrograph shows spherical,but somewhat aggregated particles having core-shell structure, withparticle size in the range 25-90 nm (see FIG. 5).

Analogously to Example 7, AR coating compositions were made by combiningthe obtained dispersion with different amounts of TEOS sol andiso-propanol; and used for preparing coated glass samples. For theproducts obtained average reflection is about 1% when cured at 650° C.,but is above about 5.8% when cured at 125° C. This difference indicatesthat at low temperature hardly any porosity is obtained, whereas at hightemperature porosity may result from calcination of organic copolymer inthe coating; demonstrating the advantage of the coating compositionaccording to the invention as prepared in e.g. Example 7 particularlywhen prepared at cured at low temperature where the emulsion stabilizeris not pyrolysed or evaporated. Results are summarized in Table 3.

TABLE 3 Amount of Average Average binder reflection (%) reflection (%)Sample (mass %) (cured at 125° C. ) (cured at 650° C. ) Example 7-1 01.7 0.8 Example 7-2 9 2.3 0.8 Example 7-3 16 2.4 0.8 Example 7-4 23 2.71.1 Example 7-5 28 3.2 1.8 Comp. exp. 8-1 0 5.8 0.8 Comp. exp. 8-2 217.1 0.9 Comp. exp. 8-3 36 5.9 0.8 Comp. exp. 8-4 45 6.0 1.2 Comp. exp.8-5 52 6.1 1.1

Humidity sensitivity: For the comparative AR coating compositions, whereonly the emulsion stabilizer and no component A is present, the minimumreflection is below 1% when cured at 650° C., but increases to above 3%at 90% relative humidity. It could be theorized (without being limitedthereto) that this may be due the mesoporosity of the coating. When ARcoating compositions according to the invention wherein component A ispresent in the nano particles are cured at 650° C. the minimumreflections stay below 1.5% even at 90% relative humidity. It could betheorized (without being limited thereto) that this may be may be due tothe reduced amount of mesopores in ARCs based on AR coating compositionscomprising core-shell nano particles with component A in the core.

Outdoor durability: After accelerated outdoor durability tests (1000hours @ 85% relative humidity and 85° C.) the AR coatings are off whitepossibly due to sodium and calcium salts having diffused out of theglass plates, but after washing with water and ethanol the AR propertiesare retained for ARCs based on the AR coating compositions according tothe invention (less than 0.5% drop in maximum transmission).

Mechanical properties: The scratch resistance of the ARCs based on theAR coating compositions 13.1-13.4 as well as on the comparative ARcoating composition pass the felt test according to EN 1096-2 with achange in transmission of less than 0.5%.

EXAMPLE 9

Several AR coating formulations were prepared by mixing core-shellparticle dispersions of Example 6 (with toluene as component A) withisopropanol and varying amounts of binder in the form of the sol of TEOSprepared as described in Example 7.

AR Coating formulation 9.1: (No binder): To 500 grams of isopropanol 6.5grams of 1:1 65% nitiric acid/water was added after which 90 grams ofthe core-shell particle dispersion of Example 6. Final pH of theformulation is 1.6 and particle size of 87 nm according to DLS. After 6weeks at room temperature the DLS value was increase less than 10 nmindication good storage stability of the particles. The formulationcontains an equivalent SiO₂ content of 1.27%.

Comparative AR coating formulation 9.2: (100% binder): Binder in theform of the sol of TEOS prepared as described in Example 7 containing anequivalent amount of silica of 4% was diluted with isopropanol to arelative amount of 1.27% SiO₂.

AR coating formulation 9.3: (35% binder): To 200 grams of the core-shellparticle dispersion of Example 6, 107.8 grams of binder in the form ofthe sol of TEOS prepared as described in Example 7 was added so a SiO₂equivalence ratio of 35/65 was obtained.

AR coating formulation 9.4: (65% binder): To 100 grams of the core-shellparticle dispersion of Example 6, 185.9 grams of binder in the form ofthe sol of TEOS prepared as described in Example 7 was added so a SiO₂equivalence ratio of 65/35 was obtained.

AR coating formulation 9.5: (90% binder): To 100 grams of the core-shellparticle dispersion of Example 6, 900.7 grams of binder in the form ofthe sol of TEOS prepared as described in Example 7 was added so a SiO₂equivalence ratio of 90/10 was obtained.

The pH of the formulations was maintained at 1.5+/−0.2 and adjusted withnitric acid if needed.

Coating formulations 9.1-9.5 were dip coated and assessed on the opticalproperties via optical transmission measurements relative to glass (typePilkington Optiwhite S; average transmission between 350 and 850 nm of91.4%). Morphology of the coatings (only dried at room temperature) wasdetermined via cross-section SEM analysis. To achieve complete dryingand hardness, the transmission was measured after 1 week.

TABLE 4 Average Cross transmission Sample section gain Observations ARcoating FIG. 6 6.02% Many core-shell particles with formulation ratherrough coating surface 9.1 Comparative FIG. 7 2.27% No pores observed.Nano pores AR coating between binder particles may be formulationpresent but are too small to be observed with this technique. Verysmooth surface AR coating FIG. 8 5.5% Core-shell particles observed andformulation some surface roughness 9.3 AR coating FIG. 9 5.05%Core-shell particles observed and formulation some surface roughness 9.4AR coating FIG. 10 4.05% Only limited number of core-shell formulationparticles observed. Smooth surface 9.5

In FIG. 11, the average transmission gain is plotted as a function ofthe origin of the silica. Each sample composition is indicated withnumber. Surprisingly, a highly un-linear behavior is observed in thatthe transmission gain remains high even at very binder contents and thatthe transmission gain only is reduced substantially when more than 90%of the silica originates from the binder.

Mechanical performance was evaluated using abrasion test performedaccording to NEN-EN 1096-2). For all formulation above >50% POT onlyminor changes (<0.5%) in transmission gain were observed after the test.Hence, even after the abrasion test, good optical properties areobtained that are of interest for commercial application in for examplethe solar cell cover glass market.

1) Process of making an anti-reflective coating composition comprisingthe steps of 1) Preparing an oil-in-water emulsion by mixing an organiccompound A; an emulsion stabilizer C; and aqueous medium of pH 2-6; at amass ratio C/A of 0.1 to 2, to result in 1-50 mass % based on emulsionof emulsified droplets of particle size 30-300 nm wherein the particlesize is the Z-averaged hydrodynamic diameter measured by DLS; and 2)Providing an inorganic oxide shell layer to the emulsified droplets byadding to the emulsion obtained in step 1) at least one inorganic oxideprecursor, to result in organic-inorganic core-shell nano-particles withmass ratio core/shell of from 0.2 to 25 where the core is the sum ofcompound A and emulsion stabilizer C and shell is metal oxide equivalentof inorganic oxide precursor, wherein compound A is an apolar organiccompound having a solubility in water of at most 5 kg/m³, and theemulsion stabilizer C is a cationic addition copolymer comprising atleast one monomer unit having a cationic charge and at least one monomerunit being neutral or non-ionic and having an overall positive zetapotential. 2) Process according to claim 1, wherein compound A is anon-polymeric compound. 3) Process according to claim 1, whereincompound A has a boiling point of at least 10° C. and at most 300° C. 4)Process according to claim 1, wherein compound A has solubility in waterat room temperature of at most 3 kg/m³. 5) Process according to claim 1,wherein copolymer C is obtained from 1-25 mole % of at least onecationic or basic monomer M1, like vinyl monomers with a tertiary aminegroup; 50-99 mole % of at least one non-ionic apolar monomer M2; and0-25 mole % of at least one polar, anionic or acidic monomer M3; withthe sum of M1, M2, and M3 adding up to 100%. 6) Process according toclaim 5, wherein copolymer C is obtained from 5-15 mole % of at leastone monomer M1; 75-95 mole % of at least one monomer M2; and 0-10 mole %of at least one monomer M3; with the sum of M1, M2, and M3 adding up to100%. 7) Process according to claim 5, wherein copolymer C is obtainedfrom 5-15 mole % of at least one monomer M1 selected from the groupconsisting of amino-functional (meth)acrylates and (meth)acrylamides;and 85-95 mole % of at least one monomer M2 selected from the group ofC1-C18 alkyl (meth)acrylates. 8) Process according to claim 1, whereinmass ratio C/A is 0.15-1.0. 9) Process according to claim 1, wherein theemulsified droplets have average particle size of 35-200 nm. 10) Processaccording to claim 1, wherein the at least one inorganic precursorcomprises an alkoxy silane. 11) Process according to claim 1, comprisinga further step 3) of adding 2-70 mass % of at least one polymeric orpolymerisable compound as binder, wherein mass % is based on the sum ofcore-shell particles and binder. 12) Process according to claim 11,comprising adding 5-50 mass % of at least one compound as binder. 13)Process according to claim 11, wherein the binder is at least oneinorganic oxide precursor. 14) Anti-reflective coating compositionobtained with the process according to claim
 1. 15) Process for makingan anti-reflective coating on a transparent substrate comprising thesteps of applying the coating composition obtained with the processaccording to claim 1 to the substrate; and drying and curing the appliedcoating layer. 16) Process according to claim 15, wherein drying andcuring is performed at a temperature of at most 250° C. 17)Anti-reflective coated transparent substrate obtained with the processaccording to claim 15.